In a radio communication system, when a mobile station selects a base station to connect to through a radio channel, the mobile station measures the transmission power of a signal transmitted from each base station and carries out control to connect to the base station having a relatively higher reception power in an effort to establish the most reliable link. If the base stations have similar signal transmission powers and coverage areas that are of a similar size, the mobile station selects the base station that maximizes the reception power at the mobile station as a counterpart for downlink communication, and selects the base station that maximizes the reception power at the base station as a counterpart for uplink communication.
In recent years, radio communication systems have improved in terms of eliminating dead zones due to limitations of radio characteristics and in terms of expanding service areas. For example, one method adopted for such improvement involves arranging small-scale base stations forming small radio cells, which are referred to as microcells or pico-cells, in the coverage area or area boundary of a base station forming a wide range cell aimed at cellular phone service. A radio communication system in which cells having coverage areas of different sizes are present together may, however, invite a situation in which the optimal radio communication link to a mobile station for uplink communication is different from the optimal radio communication link for downlink communication.
This is because the magnitude of the downlink reception power measured at the mobile station depends on propagation loss in the direction of downlink from each base station while the magnitude of uplink reception power measured at a base station depends on propagation loss in the direction of uplink, i.e., downlink reception power at the mobile station becomes greater through connection to a base station having a large-scale cell that has greater transmission power, while uplink reception power measured at a base station becomes greater at a base station having a small-scale cell that makes the distance between the base station and the mobile station shorter and propagation loss in the direction of uplink smaller.
If the mobile station determines a large-scale cell to be a connection counterpart cell according to the magnitude of reception power for receiving downlink data from the base station, although the reception quality of the radio downlink becomes optimal from the viewpoint of radio characteristics, the reception quality of the uplink becomes inferior. In contrast, if a small-sized cell is determined to be the connection counterpart cell, the reception quality of the downlink deteriorates while the radio characteristics of the uplink improve.
A method has been proposed, by which a base station and a mobile station are provided respectively with a function of independently establishing an uplink and a downlink when the optimal connection counterpart cell for the mobile station in the uplink is different from that in the downlink (see, e.g., Japanese Patent Application Laid-Open Publication No. 2007-514367). This method of connection is referred to as, for example, “uplink/downlink asymmetrical connection” or “uplink/downlink unbalanced connection”.
This method of connection uses operations of high speed packet access (HSPA) standardized by the 3rd Generation Partnership Project (3GPP), hybrid-automatic repeat request (HARQ) conforming to Long Term Evolution (LTE), etc.
In these operations, control information, such as Ack/Nack information, channel quality indicator (CQI), and Uplink Grant (UL Grant), is transmitted through a link opposite in direction to a link for data transmission, as feedback information for data transmission.
The conventional technique, however, poses a problem in that the propagation quality of feedback information for data transmission deteriorates. For example, if a mobile station receives downlink data from a first base station having a relatively high downlink propagation quality and transmits uplink data to a second base station having a relatively high uplink propagation quality, uplink feedback information for transmission of the downlink data is transmitted to the first base station having a lower uplink propagation quality than the second base station, while downlink feedback information for transmission of the uplink data is transmitted from the second base station having a lower downlink propagation quality than the first base station. As a result, the propagation quality of feedback information deteriorates.
Possible solutions to this problem include, for example, improving error correction by lowering the encoding rate of the error-correction encoding method for feedback information and increasing the transmission power for transmitting feedback information. These solutions, however, lead to another problem of an increase in communication resources and power consumption for transmission of feedback information.